Telephone system



1937. R; TAYLOR El AL 2,091,127

TELEPHONE SYS'IFEM Filed Nov. 22, 1934 9 Sheets-Sheet 1 INVENTORSREGINALD, TAYLOR GEORGE THOMAS BAKER ATTY.

Aug. 24, R T YL R r AL A 2,091,127

TELEPHONE SYSTEM Filed NQV. 22, 1934 9 Sheets-Sheet 3 I out INVENTORSREGINALD TAYLOR GEORGE THOMAS BAKER ATTY.

Aug. 24, 1937.

R. TAYLOR El AL 2,091,127

TELEPHONE SYSTEM Filed Nov. 22, ,1954

9 Sheets-Sheet 4 REGINALD TAYLOR GEORGE THOMAS BAKER ATTY.

R. TAYLOR El AL TELEPHONE SYSTEM Filed Nov. 22, 1934 9 Sheets-Sheet 5INVENTORS REGNALD TAYLOR GEORGE'THOMAS BAKER Aug. 24,- 1937.

R. TAYLOR ET AL TELEPHONE SYS TEM Filed Nov. 22, 1934 9 Sheets-Sheet 6REGINALD TAYLOR GEORGE THOMAS BAKE ATTY.

Au 24, 1937. R T R A 2,091,127

TELEPHONE SYSTEM Filed NOV. 22, 1934 9 Sheets-Sheet 7 INVENTORS REGINALDTAYLOR GEORGE THOMAS BAKER ATTY.

Aug.24, 1937. R. TAYLOR ETAL 2,091,127

TELEPHONE SYSTEM Filed NOV. 22, 1934 9 Sheets-Sheet 8 TNVENTORSREGNALD'UWLOR GEORGE'THOMAS BAKER ATTK Aug. 24, 1937. R. TAYLOR ET AL2,091,127

TELEPHONE SYSTEM Filed Nov. 22, 1934 9 Sheets-Sheet 9 SED I ob! H 28 HINVENTORS o5 REGINALD TAYLOR GEORGE THOMAS BAKER ATTY.

Patented Aug. 24, 1937 UNITED STATES PATENT OFFICE TELEPHONE SYSTEMApplication November 22, 1934, Serial No. 754,250 In Great BritainDecember 6, 1933 8 Claims.

The present invention relates to telephone systems and has for its mainobject the provision of improved automatic switching apparatus for usein such systems for increasing the rapidity and reliability with whichconnections may be set up automatically. Another object of the inventionis to provide improved methods of testing for the various conditionswhich arise during the operation of the automatic equipment.

According to one feature of the invention, a potential operated devicesuch as a thermionic Vacuum tube or a grid controlled gas discharge tubeis employed for indicating when a particular condition exists in all ofa group of pieces of apparatus. Thus it may be used for operating arelay to perform a switching operation when all of a group of pieces ofswitching equipment become busy. This feature of the invention is ofgeneral application wherever it is desired to respond to a change inpotential on a lead without connecting potential to the lead from theresponding device.

Other features of the invention, however, are concerned moreparticularly with telephone systerns employing finder switches, and fromthis point of view the invention may be considered as an improvement inthe system described in U. S. Patent No. 1,914,540. In this priorarrangement the primary finder switches were arranged so that thosefirst taken into use are directly connected to first group selectorswitches, while those last taken into use were arranged to be connectedto group selector switches in a common group over secondary finderswitches. The

primary finder switches were of the two-directional type provided withvertical and rotary magnets for effecting the setting of the switchesand with a release magnet for allowing the switch to restore to normal.

One of the features of the present invention relates to improved circuitarrangements, whereby the primary finder switches may be of thetwo-directional type in which no release magnet is provided, therestoration of the switch to nor- 45 mal being effected by theintermittent operation of one of the magnets employed for setting theswitch so that the switch wipers in the complete setting and restorationoperation perform a rec tangular movement.

50 Another feature of the invention relates to arrangements whereby thetime required in setting up the connection may be decreased by causingthe various switches to move at an increased speed. The increased speedof stepping is achieved 55 by employing self-interrupted steppingcircuits to the magnets of the various switches, together with othercircuit modifications which are necessary in order that the switchesshall still operate accurately at the higher speed.

The invention will be better'understood from 5 the following descriptionof one method of carrying it into effect, reference being had to theaccompanying drawings comprising Figs. 1 to 10; Figs. 1 to 9 should bearranged as shown in Fig.

10 to form a complete circuit. 10

Fig. 1 shows a subscribers line circuit and also the start relays SA,SB, SC, SD and SE which are common to a group of 200 lines; Figures 2and 3 show a primary finder circuit which is the same for both regularand overflow primary l5 finders. Figure 5 shows the group busy detectingcircuit for the regular and overflow groups of finders comprisingrespectively the thermionic valves VAA and VAB. The start circuits tothe primary control sets PCI, PCZ and PC3 are also shown. Figures 6, 7and 8 show primary control set PCI, which is associated with regular andoverflow finders over the distributor switches RPD and OPD respectively;Figure 9 shows one of the secondary start groups SS! which con- 25 trolstwo secondary control groups such as SCI each having access to a groupof secondary finders over a secondary distributor such as SED. Figure 4shows one of the secondary finders SFS which has access over theconnections on the 30 distributing frame IDF to finders in the overflowprimary group.

Referring now to the assembled circuit, it should be explained that thenecessary changes of connection according to the conditions under n5which the finders are to be used are made at the intermediatedistributing frame IDF shown in Fig. 4, and in this instance the brokenline connections shown are suitable for regular finders which aredirectly connectedv to first group selectors, while the connectionsshown in broken lines intercepted by dots apply to overflow finders, inwhich case connection with a first group selector is made by way of asecondary finder. The same convention is employed in connection withleads 23 and 28 in the upper right-hand corner of Fig. 5. For thepurpose of the first part of this description it will be assumed thatthe finder shown is one of a regular group, and accordingly the brokenline connections only will apply.

Referring now to Fig. 1 of the assembled circuit, when the callingsubscriber removes his receiver to originate a call, a circuit iscompleted by way of his instrument circuit over conductors l and II tooperate the line relay L over both its windings in series in circuitwith the resting armatures R2 and k3. Relay L at armature Z1 marks therotary bank of all finders with battery extended through the winding ofrelay K shunted by a comparatively low resistance l2, and at armature Z2extends battery by way of a resistance such as YA (provided one percalling level) to mark in the vertical bank multiple the level in whichthe calling line is situated. It will be seen from the drawings that thevertical bank VB Fig. 2 extends to five relays, viz: SA, SB, SC, SD andSE which are provided with two windings each, one per level, and theserelays are arranged to be operated from the marking battery extended byarmature 12 to complete a start circuit for one or other of the primarycontrol sets such as PCI (shown in Figs. 6, '7 and 8) having access byway of a distributor switch or switches to all finders in the group.

As fully described in the prior patent referred to, these relays areprovided for the purpose of starting up the primary control sets PCI,PC2 and PC3, either separately or together, depending upon the callingconditions at any particular instance. Arrangements are also shown foraltering the connections between the armatures of these relays accordingas twoor three primary control sets are provided. In the present casethree primary control sets are assumed to be fitted and accordinglyconnection is made between the terminal points U26, U28 and U30, U32 asshown. If only two primary control sets are provided the terminal U32will obviously be spare and connection is then made between terminalpoints U24, U26 and U28, U30.

- For the purposes of this description it will be assumed that relay SAis the one which is energized in parallel with the marking battery tothe vertical bank VB and at armature sal located in the bottom left-handcorner of Fig. 5 a circuit is completed for the start relay ST ofprimary control set PCI which may be traced from earth at armature sal,armature 0 23, conductor I3, armature rs2, left-hand winding of relay STto battery by way of resistance YC. Relay ST in operating at armaturesst5, silt and st! prepares a circuit for a delayed alarm changeoverdevice comprising the relays TA, TB and TC which will be describedlater, and at armature stl (Fig. 7) completes the following circuit forenergizing relay LK providing that the regular primary distributorswitch RPD has already preselected an idle regular finder: earth atoperated armature stl, comparatively low resistance l4, windings ofrelay LK in series, armatures 15b5, (Z03, tc2, T85, 0913, wiper RI andbank contact of switch RPD, conductor l5, off-normal springs NI of thepreselected finder, armature hal jumper connection at distributing frameIDF, release trunk conductor P of the associated selector to idlemarking battery therein. It should be mentioned that the preselectingcircuit of the regular primary distributor switch RPD functions in thefollowing manner. If the finder engaged by the distributor is busy,wiper RI will encounter earth connected over operated springs NI via thelow resistance right-hand winding of relay HB and relay G thereuponoperates over the following circuit: earth on wiper R5, armatures ofr3,r35, tc2, 1103, 15b5, windings of relay LK in series, resistance l4,armatures Zlcl and '012, interrupter springs rpml and 010ml, conductorI1, winding of relay G, conductor I 6, armature 0124 to battery by wayof resistance YA. Owing to its high resistance, relay G alone isoperated in this circuit and at armature g! energizes the driving magnetRPM of the regular primary distributor, in a circuit including normallyoperated armature 1722 (Fig. 5). The magnet in energizing prepares forthe advancement of the wipers Rl to R8 into engagement with the next setof bank contacts and at the conclusion of its stroke operates theassociated interrupter springs rpm], thereupon opening the circuit torelay G which releases. In this manner, interaction takes place betweenrelay G and the magnet RPM so that the wipers will be automaticallyrotated until the first idle finder is encountered.

In order to obviate the possibility of incorrect operation owing to adisconnection in the bank engaged by wiper RI it is arranged that when acall is initiated a test is made over this wiper for idle markingbattery and if no such battery is encountered, relay LK will fail tooperate and the following self-interrupted driving circuit becomeseffective tomagnet RPM to advance the wipers on to the next set ofcontacts: earth at armature stl (Fig. '7) armatures Zkl and M2,interrupter springs rpml and opml, conductor ll, armatures stZ, 0fr2 andrfz2, winding of magnet RPM to battery. Automatic rotation thereforetakes place and when idle battery is found, relay LK operates over thecircuit previously described and opens the circuit of magnet RPM at theresting contacts of armature Zlcl, while at the make contacts of thisarmature a circuit is com pleted to the upper holding winding of relayLK which is sufficiently low in resistance to mark the selected finderas busy in the bank of the distributor switch. The operation of armaturellcl also removes a shunt from relay VR which extends from itsright-hand terminal over armature d111, resistance l4, armatures Zkl and1112, interrupter springs rpm! and opml, conductor ll, armatures stZ,ofrZ, )1'22, ofc2 and $153 to its left-hand terminal. Relay VR thereuponoperates in series with the lower winding of relay LK shunted byresistance M to battery in series with the driving magnet RPM. Due tothe reversal of current flow in its lower winding the two windings ofrelay LK are now in opposition and the lower winding partly neutralizesthe flux produced by the upper winding, although the resultant fluxunder the conditions prescribed is quite sun'lcient to hold the relayoperated.

If, however, two distributor switches engage the same findersimultaneously, both relays LK in the respective control sets willoperate although one may operate somewhat before the other due to slightdiiferences which must always be present even between such similarrelays. Consequently, with two distributors testing in on to the samefinder the current in the holding winding of each LK relay will beapproximately half the normal holding current so that the opposing lowerwinding will now almost completely neutralize the effect of the upperwinding and the relays will start to release. Again due to inherentdifferences in the relays one will start to release first, andimmediately it opens its make contacts it will restore almost fullholding current to the other relay which then alone remains operated. Inorder that the relays VR shall not operate during the testing perioddescribed, they are preferably made slow to operate by providing themwith highly inductive windings which in addition are normally shunted bya resistance, and by suitable choice of values a safe period for testingcan be ensured.

When relay VR operates, it looks over its armature ml to battery inseries with the driving magnet RPM, at armature 0T4 operates relay SF(Fig. 8) in series with one winding of the line relay of the associatedgroup selector (not shown) at armature 2115 connects earth to wiper R6of the distributor to operate relay HA in the finder, at armature UTE(Fig. 7) connects relay LK in circuit with the vertical bank VB of thefinder, at armature or? (Fig. 6) connects earth to a common conductor l8to ensure that the common relay OFZ (Fig. 5) shall not operate at thistime. Relay SF in operating prepares the circuit to the cut-off relay CO(Fig. 8) which is normally short-circuited by Way of the armatures pbland pct of the wiper testing relays PA and PB. The operation of relay HAin the finder at armatures hal disconnects idle marking battery frombank R! to busy the finder and as a result relay LK in the control setreleases and at the resting contacts of its armature Zkl completes acircuit for operating relay DV over its left-hand winding. Relay DV inoperating at armature do! completes a testing circuit for relay LK, thistime in circuit with the right-hand winding of relay DV and at armaturec1112 extends earth from armatures st! and lkl to wiper R3 to complete aself-interrupted driving circuit to, the vertical magnet VM of thefinder. Intermittent energization of the magnet VM which is broughtabout by the selfinterrupting contacts 'vml, results in the shaft andwipers of the finder being raised to the marked level; when this isreached the vertical wiper VW picks up idle marking battery which isextended over wiper and bank R2 of the distributor switch to operaterelay LK over both its windings in series. Relay LK thereupon atarmature lkl opens the driving circuit to the vertical magnet andcompletes a holding circuit over its upper winding which is sufficientlylow in resistance to guard the selected level. Simultaneously with thisoperation relay LK completes a circuit to its lower opposition windin inseries with the right-hand winding of relay DV and the dual testingoperations already described are again performed in connection with thevertical bank. It will be noticed that the right-hand winding of reliefrelay DV is shunted by a resistance. the value of this resistance issuitably chosen in relation to the winding resistance so that thecombined resistance presented to relay LK is the same as that presentedwhen relay VR was connected in circuit.

Assuming that no other finder is testing on the same level at thisinstance, relay LK will hold in spite of the opposition winding and atarmature Z702 (Fig. 6) complete a circuit for operating relay RS (Fig.8) over its lower winding, armature dvd and conductor 24, in series withthe right-hand winding of the start relay ST (Fig. 6) to battery by wayof the driving magnet RPM. Owing to the high resistance of this circuitthe driving magnet RPM does not function. Relay RS in operating looksover its upper winding at armature T32 transfers the initial energizingcircuit of relay ST to the control of armature 1702, at

armature 1193 (Fig. 8) maintains relay HA in the '1 at armature hb3disconnects battery from the holding circuit of relay DV in thecontrolset which thereupon releases and completes the followingself-interrupted driving circuit to the rotary magnet RM; earth atarmature 1505 (Fig. 8) armatures m6, pal, pbl, conductor l9, armaturesd116, ofcd, wiper R8 and bank contact, conductor 28, interrupter springsrmi, winding of rotary magnet RM to battery. Intermittent energizationof the rotary magnet RM brought about due to the interrupter contactsrm! results in the wipers of the finder being rotated in search of thecalling line which may be in either the Pl or P2 bank, depending uponwhether the subscriber is located in the odd or even hundred line group.

If the subscriber is in the odd group, the test Wiper P! (Fig. 2) willencounter the marking battery when the calling line is reached and thisis extended by way of the wiper and bank R3 of the distributor (Fig. '7)to operate the test relay PA in the control set.

If, however, the subscriber is located in the even group test wiper P2will pick up the marking battery which is extended by way of wiper R5 tooperate the other test relay ,PB (Fig. 8). The operation of eithertesting relay is arranged to open the operating circuit to the rotarymagnet and thereby hold the wipers in engagement with the calling line.

If it is assumed that relay PA is the one which is operated, thenarmature pail (Fig. 8) opens the driving circuit to the rotary magnetand completes a holding circuit for relay PA over its lower winding inseries with the right-hand winding of the start relay ST to battery byway of the driving magnet RPM, and armature paE connects earth to wiperR6 of the distributor to hold the switching relay I-IA in the finder.The operation of armature pal also removes a short-circuit from thecut-off relay CO (Fig. 8) which is of comparatively high resistance andthereupon immediately operates in series with the rotary magnet RM ofthe finder. Relay CO at armature col completes a locking circuit foritself by way of the comparatively high resistance YD, at armature 003maintains relay SF on its upper winding, at armatures 004 and 005shortcircuits the upper windings of relays PB and PA respectively, andat armatures cot and col shortcircuits both windings of the start relayST which releases. Relay ST thereupon releases relays LK, VR and relayLK releases relay RS; upon the release of the latter relay earth isdisconnected from one terminal and connected to the other terminal ofthe lower holding windings of relays PA and PB at armature rst. Relay PAwhich was holding to battery from the driving magnet RPM thereuponreleases and the magnet becomes fully operated to kick the wipers of thedistributor off the now busy finder when. the operating circuit isopened. Upon the release of relay PA a short-circuit is again connectedacross relay 00 so that it too releases and in turn releases relay SF;under this condition the resistance YD limits the current in the circuitof the rotary magnet in the finder to prevent its further operation. Allrelays in the control set are now restored to normal and upon theopening of the circuit of the driving magnet RPM at armature cat themagnet de-energizes and the wipers of the distributor switch areautomatically advanced on to the next set of contacts.

Relay K in the calling subscribers line cir-- cuit operates in serieswith the test relay in the control set and when the control set releasesit locks in series with the switching relay in the finder over a circuitwhich may be traced as follows: battery via the winding of relay K,operated armature kl, release trunk conductor P, bank and wiper Pl ofthe finder, armatures M3 and 72.123, springs NRE, conductor 2!, upperlowresistance winding of relay HA, conductor P to earth connected up atthe controlling switch in the train. Relay K also at armatures 702 andk3 clears the calling circuit of its battery and earth connections,whereupon relay L releases after its slow period and opens the markingcircuit and the start circuit to the primary con- 5 trol sets. It willbe noticed that upon the release of armature ll, relay K is againconnected to the private nermal conductor extending to the finalselector bank multiple, but owing to the connection of comparatively lowresistance earth through the upper winding of relay HA the circuit iseffectively guarded against selection.

The calling circuit is now extended by way of the wipers negative 5,positive I, Pi and M! of the primary finder over resting armatures ofrelay HB and operated armatures of relay HA over the dotted connectionsat the distributing frame IDF to the associated first selectoraccessible over the bracketed conductors (Fig. 4). It may be howeverthat both test wipers Pi and P2 will encounter marking battery fromdifferent calling lines at the same instant, and in this case relay PAin operating will take precedence over relay PB since the lockingcircuit of the latter includes a resting contact of relay PA. Operationof the calling subscribers meter MTR in Fig. 1 is effected when thecalling subscriber replies by the application of potential to themetering conductor M by the controlling switch in the train.

Upon the release of the connection when the 40 calling subscriberreplaces his receiver, holding earth is removed from the release trunkconductor P by the controlling switch in the train, whereupon therelevant switching relay in the finder and the K relay in the linecircuit re- 45 lease. The release of the switching relay in the findercompletes a self-interrupted driving circuit to the rotary magnet RMfrom earth connected to conductor 22 by way of common equipment arrangedto give an alarm in case of fail- 50 ure to release and the wipers arethereupon automatically rotated to the 12th position. In this positionthe normal rotary springs NRI restore and the supporting means for theshaft and Wipers is mechanically tripped so that they fall 55 by gravityassisted by a spring and then rotate beneath the bank under the: controlof this spring to the full normal position. The construction of atwo-directional switch arranged to restore to normal in the manner justdescribed is given in 60 British Patent No. 391,123.

In the fully normal position the ofi-normal springs N! and N2 againopen, springs NI disconnect low resistance earth by way of the righthandwinding of relay HB from the bank R! of 65 the distributor switch andreplacing it by battery from the first group selector to mark the finderas idle. Springs N2 open the self-interrupted driving circuit to therotary magnet and the magnet ceases to function.

It will be appreciated from the circuit arrangement that the operationof the rotary magnet persists during the period that the wipers arebeing restored vertically and moved to their normal position below thebank, and it is therefore 75 conveniently arranged that the rotary pawlis out of engagement with the ratchet during this period. If the switchfails to release within a reasonable period, the continuous operation ofthe rotary magnet will result in the common release signal equipmentfunctioning after a suitable period to give an alarm of the faultycondition.

If the calling subscriber should abandon the call during the findingoperation, the start relay ST will release either due to the release ofthe level start relays SA to SE if armature rsZ is not operated, or dueto the release of relay LK if armature 1'82 is operated. It will berecollected that relay BS is operated when the calling level is found sothat if the calling subscriber abandons the call before the callinglevel is found relay ST will release due to the release of the levelstart relays while if the calling subscriber abandons the call after thecalling level is found relay ST will release due to the release of relayLK. Relay LK in these circumstances is holding over its upper winding tomarking battery connected over the vertical bank so that when themarking battery is removed due to the release of relay L then relays LKand ST will release. The release of relay ST causes the control set andthe finder to restore to normal and it will thus be seen that wheneverthe calling subscriber hangs up his receiver the release of the finderimmediately takes place.

Consideration will now be given tothe case when all regular finders ofthe group in question are in use so that further calls will require tobe extended over and overflow finder having access to a selector in thecommon group by way of a secondary finder such as SFS (Fig. 4) Owing tothe fact that all the regular finders are in use negative markingbattery will disappear from the common conductor 23 (Fig. 5) extendingto the grid of the valve VAA whereupon the grid of this valve thenbecomes positive with respect to the filament and as the result of a.substantial increase in the plate circuit current produced by thiscondition relay RFB connected in the plate circuit then operates. Thedevice VAA is preferably a vacuum tube having three or more electrodes,but use may alternatively be made of a grid controlled gas dischargetube or the like. It will be understood that the metal rectifier MRAprovided in each finder is connected up in such sense as to prevent theearth subsequently connected tothe bank RI of the distributor fromshunting the idle marking battery of other finders connected to the samecommon conductor 23. Relay RFB in operating short-circuits relay RFZwhich it will be remembered is normally operated in series with thefilament of the valve VAA so that this relay releases. Relay RFZ atarmature rfzl completes the filament circuit to the other valve VAB byway of the various resistances shown, at armature rfzZ removes theshort-circuit from the upper winding of relay OFC (Fig. 6) of primarycontrol set PCI, at armatures H23 and 1724 removes a similarshort-circuit from relays CFO of primary control sets P02 and PCS showndiagrammatically as dotted rectangles, at armature rfeE prepares alocking circuit for relay CFO of primary control set PCI, at armatures1726 and Tfel prepares similar locking circuits for relays OFC inprimary control sets P02 and P03 and at armature rfzB (Fig. 9) connectsup earth to operate relay OS. One relay, such as OS is provided to everythree secondary start groups such as SSI (Fig. 9) and controls in all 6armatures, two of which are associated with each secondary start groupthat is, one to each of the two control circuits associated with eachsecondary start group. One of the armatures only is shown, namely 08!(Fig. 9) which on operation disconnects earth from the bank contacts ofthe overflow primary distributor OPD and replaces it with idle markingbattery by way of the resistance YA connected in parallel with the upperwinding of the start relay DS of secondary start group SSl. The otherarmature of relay OS associated with the secondary start group SSi islocated in the other secondary control circuit.

In case regular and overflowfinders were connected to the samedistributor switch, earth connected over resting contacts of relay OSwould serve to mark the overflow as busy while regulars were stillavailable.

Returning again to relay OFC in the primary control set PCi, it shouldbe explained that although the short-circuit is removed from its upperwinding this relay does not operate until the last regular finder hasbeen fully set and the control relays are restored to normal. When thisis done the driving magnet RPM of the regular primary distributor RPD isenergized in the kickon circuit to advance the wipers on to the next setof bank contacts, but as all regular finders are now in use testingwiper RI will encounter earth in all positions of the bank and thepreselecting control relay G thereupon immediately operates. Relay G atarmature g! extends earth to the driving magnet RPM but as the armaturerfzi. has opened meanwhile, the circuit is completed via the upperwinding of relay O-FC which thereupon operates and is of sufficientlyhigh resistance to prevent the operation of the magnet. Relay OFCthereupon locks on its lower winding by way of armature ofci and in aparallel circuit operates the relief relay OFR.

It will be seen from the drawings that the operation of relays CFO andOFR at armatures ofcZ, ofc3, o-fc i, 0 05, ofcS, 0 1'2, o-fr3, 0 74, 0W5and 0 1'6 transfers the control leads extending from primary control setPM to the driving magnet and wipers of the overflow primary distributorOPD.. Similar operations take place in primary control sets P02 and PC3so that their associated overflow primary distributors are con.- nectedin circuit and all subsequent calls in the group are thereupon completedover overflow finders.

For the purpose of the description, it will now be assumed that afurther call is handled by primary control set PCI and that the findershown in Figs. 2 and 3 is one of the overflow group. It will also beunderstood that the connections shown in broken lines intercepted bydots will now apply. Relay ST is operated from the start circuit in themanner already described, and as the overflow primary distributor OPD isalready standing in engagement with a free overflow finder, wiper 0!will pick up idle marking battery by way of the connections at thedistributing frame to the resistance YA connected in parallel with startrelay D8 of the secondary start group SS! (Fig. 9). Relay LK in theprimary control set PC! thereupon immediately operates and locks, relayDS also operates in this circuit. After a short period relay VR operatesto prepare for the vertical operation of the finder in the mannerpreviously described. Under the present circumstances, however, andowing to the different connections at the distributing frame, relay VRalso at armature m3 extendsdirect earth over the Wiper 01 of thedistributor tomaintain the start circuit to relay DS in the secondarystart group and also to busy this point of access to other huntingdistributor switches. Earth at armature 0T3 also provides a lockingcircuit over the metal rectifier MRB for relays OFC and. CPR which iseffective in case a regular finder becomes free meanwhile and bringsabout the re-operation of relay RFZ. The metal rectifier MRB is includedin this circuit so as to prevent earth from armature 7725 starting upthe secondary control group prematurely.

It will be understood that the secondary start groups SS! are accessiblein common from the overflow primary distributor switches servingdifferent groups of lines and are provided in numbers sufficient tocater for the maximum trafiic on these groups of lines. It is alsoarranged that adjacent contacts in the banks of the distributor switchesextend to different secondary start groups in order to increase thepossible availability, although it is conceivable that the samesecondary start group may make more than one appearance in the banks ofa distributor switch but only on contacts which are widely spaced apart.

Returning again to the primary control set PC! it will be seen thatrelay VR at armature vr i (Fig. 8) again connects up relay SF, but underthese circumstances relay SF is not immediately operated and theresistance YA connected in parallel serves to connect up a markingbattery to mark the overflow finder which has been taken into use in thebank multiple of the secondary finder switches SFS. From this point theoperations of the overflow finder proceed as described for the regularfinder and when the calling line is located one or other of the testingrelays PA or PB in the primary control set operates toterminate thehunting operation. If the overflow finder taken into use is not yetfound by one of the secondary finders SFS there will be no circuit forrelay CO owing to the non-operation of relay SF and the control settherefore remains held in this condition.

It will be understood that simultaneously with the, hunting operation ofthe overflow finder to find the callin'glline, two secondary finderssuch as SFS (Fig. 4) are set in operation to find the overflow finderwhich has been taken into use. These secondary finders have no normalresting position for their wipers and it will be appreciated therefore,that the arrangement of setting two in operation simultaneouslyconsiderably reduces the time taken to find the overflow finder.

Considering the detailed circuit operations, when the start relay DS isoperated from the primary control set it locks over its armature dsl tothe access point S2 and at armatures ds3 and (Z35 completes a circuit tothe control relays of the secondary distributor switch SED to cause thisswitch to select the first available secondary finder. A similar circuitis completed by way of armatures ds4 and 1186 to another secondarydistributor (not shown) which also selects a further secondary finder.In order to still further reduce the finding time it is arranged thatthe secondary distributors perform a preselecting function which ensuresthat their wipers. are automatically positioned into engagement with afree secondary finder if there is one available.

. It will be seen from reference to the circuit that under normalconditions .if the secondary. finder engaged by the'distrib-utor .isbusy, the

test wiper SEi will encounter earth from armature ski. which is operatedat this time, and this is extended through both windings of relay DK inseries, comparatively low resistance 25 shunted by the high resistancerelay DR and another resistance 26, interrupter springs semi, armatures(Z83, sdi and ,fTB, winding of extremely high resistance relay iG tobattery. Owing to its high resistance, relay iG is only operated in thiscircuit and at armature igi energizes the driving magnet SEM of thedistributor. The magnet SEM in energizing prepares for the advancementof the wipers SE! to SE5 into engagement with the next set of bankcontacts and at the conclusion of its stroke operates the associatedinterrupter springs semi, thereupon opening the circuit to relay iGwhich releases. In this manner interaction takes place between relay iGand the magnet SEM so that the wipers are automatically stepped intoengagement with a free sec ondary finder. It therefore follows that assoon as relay DS operates, both secondary distributors SED should switchthrough on to idle secondary finders but in order to obviate thepossibility of incorrect operation owing to a disconnection in the bankengaged by wiper SEI it is arranged that when a call is initiated a testis made over this wiper for idle marking battery connected to the P wireby the associated group selector (not shown). If no such battery isencountered relay DK will fail to operate and the followingself-interrupted driving circuit becomes effective to the magnet SEM toadvance the wipers on to the next set of bank contacts; earth atarmature c185, armatures dki and dr3, interrupter springs semi, armatureds3, winding of the magnet SEM to battery. Automatic rotation thereforetakes place and when an idle marking battery is found relay DK thenoperates over both its windings in series and opens the circuit of themagnet at the resting contacts of its armature dlci, while at the makecontacts of this armature a circuit is completed to the holding windingof relay DK which is sufficiently low in resistance to mark the selectedsecondary finder as busy in the bank multiple of the distributorswitches SED. The operation of armature dlci also disconnects theshunting resistance 25 from relay DR which thereupon proceeds to operatein series with the left-hand winding of relay DK which is shunted by theresistance 25, to battery in series with the driving magnet SEM. It willbe recognized that relays DK and DR present a similar combination torelays LK and VR 5 in the primary control set and they are intended toperform an exactly similar function, namely the prevention of dualswitching.

If it is assumed that no other secondary allotter SED is testing thesame contact, relay DK will hold in spite of the opposition of itsleft-hand winding and after a short interval relay DR also operates.Relay DR locks by way of its armature dri, at armature drZ disconnectsthe shunting resistance 26 at armature dr3 opens 5 a further point inthe circuit to the magnet SEM, at armature drd connects full earth towiper SEi at the same time short-circuiting relay DK which releases, atarmature dr5 extends earth over wiper SE5 to the negative conductor toprepare the associated group selector for operation, at

armature drfi prepares the testing circuit to relay FK and at armaturedri extends earth over wiper SE3 to energize the driving magnet SEM. Thedriving magnet of the secondary finder thereupon energizes in aself-interrupted circuit controlled by the contacts sfmi to rotate thewipers SF i to SF4 in search of the overflow finder which has been takeninto use. When this is found the test wiper SFi will pick up markingbattery connected in parallel with relay SF in the primary control setwhich is extended over the bank and wiper SE4 of the secondarydistributor switch to energize relay FK over both its windings inseries. Relay FK thereupon opens the circuit of the secondary findermagnet at the resting contacts of its armature flci, while at the makecontacts of this armature a circuit is completed to the upper holdingwinding of relay FK which is sufficiently low in resistance to mark theoverflow finder selected as busy in the bank multiple of the secondaryfinder switches SP8. The operation of armature fki also removes a shuntfrom relay FR which thereupon proceeds to operate in series with thelower winding of relay FK shunted by a resistance 21 to battery inseries with the driving magnet SFM. Again the relays PK and FR aresimilar in function to relays DK and DR in that they guard against thedual switching of secondary finders on to the same overfiow finder.

If it is assumed that no other secondary finder is testing on the samecontacts at this instant, relay FK will hold in spite of the oppositionof its lower winding and relay FR thereupon operates after a slightinterval, thereby at armature fri disconnecting the opposition windingto relay FK, at armature fr2 supplements the earth to wiper SE5, atarmature ,fr3 opens the shunt about its own winding, and at armature,fr5 provides an alternative holding circuit to relay DR. Relay SF inthe primary control set is operated in series with relay FK in thesecondary distributor, and at armature sf2 completes an operatingcircuit for relay 00 which becomes immediately effective since it isassumed that the overflow finder has already found the calling line.Relay SF also at armature sfi opens the circuit to the start relay DS inthe secondary start group and at the make contacts of this armatureconnects earth by way of the metal rectifier MRB to hold operated thechangeover relay OFC and CPR for the purpose already described. Relay COserves to release the primary control set and allow the overflow finderto switch through in the manner already described and when this is donethe holding circuit to relay FK in the secondary distributor is openedand it releases. A circuit is thereupon completed at armature flci foroperating the switching relay SK in the secondary finder which looks byway of its armature ski to earth connected to the release trunkconductor P from the associated group selector, at armature s7c2connects guarding earth to the bank of the secondary distributor, and atarmatures slc3 to skfi extends the negative, positive, M and P leadsthrough to the associated group selector.

The calling subscriber now receives dialling tone from the groupselector and proceeds to set up the connection by operating his dialswitch in the usual manner.

Relay iG is operated from earth extended over the front contact ofarmature ski, wiper SE2 and armatures frd and fr6; and at armature i gienergizes the driving magnet SEM to prepare for the advancement of thewipers of the distributor switch on to the next set of contacts. RelayDR is short-circuited by earth connected up by armature i gi whereuponit releases and is followed shortly afterwards by relay FR which in turnopens the circuit to relay IG. All relays in the secondary control groupare therefore released and upon the de-energization of the magnet SEMthe wipers are automatically stepped on to the next set of contacts.

Considering now the condition when the last overflow finder in the groupis taken into use, negative battery will be removed from the commonconductor 28 extending to the valve VAB 10 (Fig. 5) whereupon the gridof this valve then becomes positive with respect to the filament and theincrease in the plate current brought about by this condition results inthe operation of relay OFB. Relay OFB at armature ofbl com- 15 pletes acircuit for relay OFZ which becomes effective when the last overflowfinder has been fully set and the primary control set restores tonormal. When this happens relay VR in the control set releases and atarmature v'rl' removes 20 a short-circuit from relay OFZ whichimmediately operates and at armatures ofzl, ofzZ and ofz3 transfers thestart circuit for the various primary control sets to conductors 29, 30and 3| extending to overflow meters (not shown) in- 25 dividual to eachset. Further calls during this condition are therefore registered on theparticular overflow meter concerned. In case an overflow finder shouldbecome free meanwhile, the grid of the valve VAB will again become 30negative and as a result of the decrease in the plate current, relayOFBwill release to open up access to the primary control sets.

Similarly, in case a regular finder should become free meanwhile, thegrid of the valve VAA will become negative whereupon relay RFB releasesto open up the primary control sets for access to the regular findergroup.

If all secondary finders under the control of the start group SSI (Fig.9) become busy, relay 40 OB will release due to the removal of all idlebattery connections from the common conductor 32. Relay F3 is operatedfrom armature obl and at armatures fbl and fb-Z opens the drivingcircuit to the respective secondary distributors to 45 prevent uselesshunting, and at armatures fb3 and b4 extends earth to the access pointsSI and S2 and disconnects the start relay B8.

In case the secondary start group shown should be the last in use priorto the busy condition, it

50 will be seen that the release of armature ob2 bridges both accesspoints SI and S2 to ensure that overflow finders connected to one accesspoint are immediately busied in the banks of the distributor switchesOPD, from the overflow find- 55 er in use connected to the other accesspoint. It is conceivable that the condition may arise when all secondaryfinders in the exchange are in use at the same instant. This will havethe effect of restoring all the OB relays in the exchange and 60consequently negative potential will be removed from the grid circuitsof all valves VAB in the exchange whereupon the respective relays areoperated to connect up the overflow meters and prevent access to theoverflow finders.

5 As mentioned in the early part of the description relay ST (Fig. 6) inoperating is arranged to connect up the delayed alarm changeover setwhich functions in case the primary control set is not freed within apredetermined period of 7 time.

Considering now the operation of the changeover set in detail, relay STin operating at armature st extends earth over common conductor 33 whichsets in operation a motor arranged to 75 drive the cam assembly 34 whichapplies an earth impulse to the S and Z conductors having a definiteperiod of time between each application. When earth is applied to the Spulse conductor a circuit is completed for operating relay TA over itsleft-hand winding whereupon the relay locks on its right-hand winding byway ofarmature tal, at armature m2 connects relay TB in circuit with theZ pulse conductor, and at armature m3 prepares a short-circuit to thelefthand winding of the start relay ST.

If the control equipment fails to release after the predetermined periodhas elapsed an impulse will be delivered overthe Z pulse conductor whichoperates relay TB over its left-hand winding so that this relay thenlooks over its right-hand winding at armature tbl, to earth at restingarmatures similar to tb'Z in primary control sets P02 and PC3. cuit anindividual fault alarm lamp LP is lighted, at armature tb2 earth isextended over conductor 35 to actuate an alarm associated with theparticular rack upon which the equipment is mount ed, at armature weearth is extended over another common conductor 36 to set in operation amain alarm which is common to the exchange, at armature tb l theshort-circuit is removed from relay TC (Fig. 8) at armature tb5 relay LKis released,- at armature tbfi the left-hand winding of the start relayST is short-circuited and released and at armature tb'l the startcircuit is transferred to primary control set PC3. The primary controlset PC! is thus released and switched out of service and further callsthereto are handled by primary control set PC3.

If, however, the primary control set has become associated with afinder, one or other of the relays VR, BS or PA will be operated andtherefore when the short-circuit is removed from. relay TC (Fig. 8) itimmediately operates in series with relay HA in the finder. Relay TClocks by way of its armature tcl, at amature tc2 extends earth to thetest wiper R! of the distributor if relay RS has not operated to busythe finder against further selection, at armature tc3 opens the circuitbetween relay PA and the test wiper PI of the finder since otherwise theoperation of armature rs? would guard the line selected and prevent thesubscriber from being served by other finders, at armature ted releasesrelay VR. and maintains relay RS if operated, at armature 1505 (Fig. 8)opens the circuit to the rotary magnet of the finder, and at armaturetot (Fig. 6) provides a circuit for maintaining relay OFR. in case afault has occurred during overflow working. In this manner, therefore,the faulty circuit is looked as busy until the fault is cleared,whereupon the attendant will momentarily open the fault release link FRL(Fig. 6) to open the locking circuit of relay TB and restore the circuitto normal.

In case the regular first group selector is artificially busied for testpurposes by the connection of earth to the release trunk conductor P,this earth will find a circuit by way of the resting oif-norma1 springsN! in the finder to the testing bank RA of the distributor and thereforemarks the associated finder as busy. i

If the first group selector associated with secondary finder isartificially busied for test purposes earth connected to the releasetrunk conductor P serves to busy the secondary finder concerned and alsoshunts the idle marking battery insofar as its function in holding relayOB is concerned, although it will be appreciated that owing to thepresence of resistance YA in the cir- In parallel with this lockingcircuit, relay OB will remain held from idle marking batteries of otheridle secondary finders in the same group.

We claim:

1. In a telephone system, calling lines having terminals arranged ingroups, a finder switch having access to said terminals, said finderswitch operated responsive to a call over one of said lines to stepvertically to a group of said terminals and then operated in a rotarydirection to find the terminal of the calling line, and means forcontinuing the rotary movement in the same direction to initiate therelease of the switch.

2. In a telephone system, calling lines having terminals arranged ingroups, a finder switch having access to said terminals, a vertical anda rotary magnet for operating said switch responsive to a call over oneof said lines, said vertical magnet operating the switch vertically tothe group of terminals in which the calling line is located, and saidrotary magnet then operating the switch in a rotary direction to findthe terminals of the line, and means for operating the rotary magnetagain to initiate the release of the switch, and a self-interruptedcircuit for said rotary magnet over which the rotary magnet is operatedin both movements.

3. In a telephone system, an automatic hunting switch, lines accessiblethereto, said switch having movement in two directions to find one ofsaid lines, a common apparatus for controlling the movement of saidswitch, magnets in the switch for operating the switch in said twodirections, and means for operating one of the said magnets to releasethe switch independent of the control by the common apparatus.

4. In a telephone system, a switch as claimed in claim 3 in which thereis a self-interrupted circuit for said one magnet and means foroperating said one magnet over said circuit both in the setting andrelease operations.

5. In a telephone system, a plurality of regular trunk circuits, aplurality of overflow trunk circuits, an electric discharge tube havinga grid circuit, a source of potential in each of said regular circuitsconnected to said grid circuit, means in each of said regular circuitsfor varying the potential connected to the said grid circuit, andapparatus in the output circuit of said tube controlled bychanges in allof said circuits connected to the input of the tube for rendering theoverfiow circuits effective.

6. In a telephone system, a plurality of circuits, an electric dischargetube, an input circuit for said tube connected to all of said circuits,means controlled over each circuit for disconnecting that circuit fromthe input of said tube, an output circuit for said tube, and apparatusin said output circuit operated when all of the circuits aredisconnected from the input circuit of the tube.

7. In a telephone system, a plurality of switching devices, distributorswitches operating to find an idle switching device, an electricdischarge tube, means in each device for maintaining a potential on thegrid of said tube as long as the devices are idle and for removing saidpotential when the devices are all busy, and means in the output circuitof said tube responsive to the removal of said potential for preventingthe operation of said distributor switches.

8. In a telephone system, a group of regular finder switches and a groupof overflow finder switches, an electric discharge tube, a lead fromeach regular finder to the grid of said tube, and means in the outputcircuit of said tube responsive to a change in potential in each of saidleads for making the overflow finders available I when all the regularfinders are busy.

REGINALD TAYLOR. GEORGE THOMAS BAKER,

